The recent technology transition from film to “electronic media” has spurred the rapid growth of the imaging industry with applications including still and video cameras, cell phones, other personal communications devices, surveillance equipment, automotive applications, computer based video communication and conferencing, manufacturing and inspection devices, medical appliances, toys, plus a wide range of other and continuously expanding applications. The lower cost and size of digital cameras (whether as stand-alone products or imbedded in other appliances) is a primary driver for this growth and market expansion.
Although traditional component manufacturers continue to shrink the components to take advantage of the electronic media, it is difficult to achieve the ever tightening demand of digital camera producers for smaller sizes, lower costs and higher performance. Several important issues remain, including: 1) the smaller the size of a digital camera (e.g., in cell phones), the poorer the image quality; 2) complex “lenses”, shutter and flash are still required for medium to higher quality imaging, thus negating much of the size advantage afforded by the electronic media; and 3) the cost advantage afforded by the electronic media is somewhat negated by the traditional complex and costly lens systems and other peripheral components.
Most applications are continuously looking for all or some combination of higher performance (image quality), features, smaller size and/or lower cost. These market needs can often be in conflict: higher performance often requires larger size, improved features can require higher cost as well as a larger size, and conversely, reduced cost and/or size can come at a penalty in performance and/or features. As an example, consumers look for higher quality images from their cell phones, but are unwilling to accept the size or cost associated with putting stand-alone digital camera quality into their pocket sized phones.
One driver to this challenge is the lens system for digital cameras. As the number of photo-detectors (pixels) increases, which increases image resolution, the lenses must become larger to span the increased size of the image sensor which carries the photo detectors. The pixel size can be reduced to maintain a constant image sensor and optics size as the number of pixels increases but pixel performance is reduced (reduced photo-signal and increased crosstalk between pixels). Also, the desirable “zoom lens” feature adds additional moveable optical components, size and cost to a lens system. Zoom, as performed by the lens system, known as “optical zoom”, changes the focal length of the optics and is a highly desired feature. These attributes (for example, increased number of pixels in the image sensor and optical zoom), although benefiting image quality and features, may adversely impact the camera size and cost. In some cases, such as cell phones or other appliances where size and/or cost are critical, this approach to good image quality (high resolution and sensitivity) is not optimum.
Digital camera suppliers have one advantage over traditional film providers in the area of zoom capability. Through electronic processing, digital cameras can provide “electronic zoom” which provides the zoom capability by cropping the outer regions of an image and then electronically enlarging the center region to the original size of the image. In a manner similar to traditional enlargements, a degree of resolution is lost when performing this process. Further, since digital cameras capture discrete input to form a picture rather than the ubiquitous process of film, the lost resolution is more pronounced. As such, although “electronic zoom” is a desired feature, it is not a direct substitute for “optical zoom.”
Conventional digital cameras typically use a single aperture and lens system to image the scene onto one or more image sensors. Color separation (if desired), such as red, green and blue (RGB), is typically achieved by three methods: 1) a color filter array on a single integrated circuit image sensor, 2) multiple image sensors with a color separation means in the optical path (such as prisms), or 3) an imager with color separation and multiple signal collection capability within each pixel. These three color separation method have limitations as noted below.
The color filter array, such as the often used Bayer pattern, changes the incident color between adjacent pixels on the array and color crosstalk occurs that prevents accurate color rendition of the original image. Since the array is populated with pixels of different color capability, interpolation techniques are required to create a suitable color image. The color filter array may also have low and variable optical transmission that reduces received optical signal levels and creates pixel-to-pixel image non-uniformity.
The use of multiple imagers, with color separation methods such as a prism, provides accurate color rendition but the optical assembly is large and expensive.
Color separation methods within the pixel create crosstalk of colors and inaccurate color rendition. Since multiple color charge collection and readout means are required in each pixel, pixel size reduction is limited.
Technology advances in lenslet optical design and fabrication, integrated circuit imager pixel size reduction and digital post-processing have opened new possibilities for cameras and imaging systems which differ dramatically in form fit and function from time-honored digital camera designs. The use of multiple camera channels (multiple optics, image sensors and electronics) in a compact assembly allows fabrication of a digital camera with improved image quality, reduced physical thickness and increased imaging functionality not achievable with a conventional single aperture/optical system digital camera architecture.